1. Field of the Invention
This invention relates to a system for producing an image of a document and more particularly to a system which provides non-uniform illumination of the original document varying with the image density at different locations on the document.
2. Description of the Prior Art
Conventionally, images of large construction drawings are produced by utilizing a large planetary camera, which is well known in the art, such as in the case of microfilming. A drawing is placed on an illuminated table or easel and a frame of photosensitive material, such as film, is exposed through a timed shutter. The film may be advanced to the next frame and the next drawing to be microfilmed placed on the illuminated easel. Alternatively, a moving lens may be used to optically arrest the motion of documents traveling in a high speed stream for photographic exposure onto stationary, incrementally advanced photographic film, such as is shown in T. W. Howard, U.S. Pat. No. 4,374,618.
The density of the image produced on the photosensitive medium is related to the level of light reflected from the document being recorded. It is well known in the art that documents may be illuminated from either above or beneath, especially in the case of transparent or translucent originals. E. Hahn, et al., U.S. Pat. No. 3,947,112 shows the use of incandescent illumination from above the documents and fluorescent illumination from below. In current microfilming systems, a reflected light meter is customarily positioned above an area of a drawing that the photographer considers representative of the overall background density of the drawing. The overall illumination of the easel is then adjusted to produce the requisite average amount of reflected light at the area being measured, depending on such factors as film sensitivity, original document contrast, and image density.
Large construction drawings generally have severe variations in image density, due to the presence of both large open areas and areas having a predominance of fine line data. Thus, with prior art microfilm systems it is usually necessary to record the drawing on several film frames having different illumination levels in order to capture all the required detail. This makes the viewing of the information cumbersome to the user since he or she must use several frames to examine all areas of the drawing. This limitation further makes it difficult or impossible to print the entire drawing back from the microfilm since no single frame of microfilm contains an image of the entire drawing.
Another problem which has long prevented the efficient and smooth operation of high speed document recording and reproducing systems is the manner and extent to which the original documents must be handled. Large documents are generally laid flat on a table just prior to being fed into the image producing system. After being photographed, copied, or otherwise reproduced, the document generally emerges from output means of the machine, at which time it is usually again laid flat on a separate pile of completed documents. Such handling often results in torn or damaged documents, reduces the efficiency of a machine which otherwise might have a greater capacity for high speed document recording, and wastes a good deal of space around the imaging system, the wasted space being used only to pile up completed documents.
One aspect of this invention is based on the problem of highly variable image density on large format construction drawings. Another aspect of the invention is directed to improving handling of original documents after filming, also known as document off-loading.